If you only use 5 players, then each player will attempt to play the entire game. Even when a player gets tired, he will stay in the game, although you can expect a drop in performance.
There are two things that can force one of your players out of the game; an injury, or fouling out. If either of these things happens, then the player will be replaced by a lucky fan, who you can expect to play very badly. Whenever your team has fewer than 5 players available to put into a game, then lucky fans will fill any of the empty spots in the lineup.